Sign in with Facebook

- Added gem 'omniauth-facebook' to gemfile.
- New features: sign in and out using facebook.
- User model added.
- Specs for login, logout and User model.
- Setup facebook_app_id and facebook_secret tokens using env vars.
Facebook Sign In refactoring

- Specs refactoring. OmniauthHelpers module added.
- Minor code corrections
Merge pull request #6 from insignia4u/facebook_login

Sign in with Facebook